Optical sights are used for various purposes, one example of which is mounting a sight on a weapon in order to help a user accurately aim the weapon. The optical sight takes image information from a distant scene, and presents this image information within a field of view which is visible to the eye of a user.
In reflex gun sights, a positioning marker or image, such as cross-hairs, a luminous dot, or other type of reticle is superimposed onto the visible field of view to aid a marksman in aiming the weapon and hitting the target within the field of view. The positioning marker can be generated using a light source such as a light emitting diode (LED). Changes in one or more physical properties of the light source, such as drive current or temperature, can cause a spectral shift in the light emitted from the light source. Additionally, individual units of the same nominal light source can have spectral shifts relative to the nominal source. This shift can produce undesirable effects such as changes in perceived brightness or a shift in the hue of the light. A change in perceived brightness is due to the eye's variable sensitivity to the color, or wavelength, of the light. As the emitted wavelength spectrum of the light source shifts toward wavelengths to which the human eye is more visually sensitive, the eye perceives this shift as a brightening of the light source. As the emitted wavelength spectrum of the light source shifts toward wavelengths to which the human eye is less visually sensitive, the eye perceives the shift as a dimming of the light source. These spectral shifts can also influence the perceived hue of the light source. The effects of spectrum shift also occur when mixing multiple color light sources to create computer or other visual displays. In a reflex gun sight, the effects of spectral shift can cause the reticle to appear too bright, distracting the user from the field of view, or too dim, causing the user difficulty in discerning the reticle against the field of view.
New apparatus and methods are needed to overcome the effects of spectral shifting.